The present invention relates to an automated sample handling system having a heated liquid sampling probe.
Automated sample handling systems are known which automatically dispense fluid samples, such as blood plasma and reagents, into the reaction well of a cuvette. Such instruments are useful in the field of biochemical analysis for measuring blood clotting times and for automatically carrying out other bioassays.
An automated sample handling system of this type is described in U.S. patent application Ser. No. 07/443,951. In this system a temperature controlled housing is provided for storing the fluid samples and reagents at a relatively cool temperature for preventing degradation of the samples and reagents prior to sample analysis. The temperature controlled housing typically maintains the fluid samples and reagents at a temperature of 4.degree. C. The actual analyses are generally carried-out at 37.degree. C. (98.6.degree. F.), standard human body temperature. Accordingly, it is necessary to heat up the fluid sample and reagents to 37.degree. C. prior to analysis.
In the above-described sample handling system, a temperature-controlled linear track on which the cuvettes are transported is sufficient to bring the fluid samples to the reaction temperature. However, it has been found necessary to pre-heat the reagents while in the sampling tube prior to being dispensed in the reaction well of a cuvette in order to assure the reaction volume is at the proper temperature at the proper time.
In order to achieve the desired heating of a volume of liquid in the liquid sampling tube, it has been attempted to use an electric heating element disposed directly on the tube. These attempts have failed because previous applications of electrical heating elements placed directly against the sampling tube have caused malfunctions in the capacitive touch liquid level sensing devices commonly used with automated liquid sample handling systems.
One of those known attempts to heat the sampling tube and the aspirated liquid therein by an electric heater element was abandoned owing to unsatisfactorily results. The results were unsatisfactory because the electrical heater increased the capacitance developed between the liquid sampling tube and the instrument chassis to such a large extent that the capacitive touch liquid level sensing device or, simply, the capacitance measuring device, was unable to accurately detect the change in capacitance when the sampling tube contacted the surface of the liquid to be sampled. Thus, for example, a robotic arm controlled by information from the capacitance measuring device failed to stop lowering a sampling tube into a liquid sample as the capacitance measuring device was unable to detect the surface of the liquid to be sampled and, hence, was unable to send such information to a central controller for operating the robotic arm.
To understand why those known attempts at using an electrical heating element disposed directly on the liquid sampling tube failed, a discussion of how the capacitance measuring device functions, as applied in this environment, follows.
Basically, in this environment the capacitive liquid level sensor works on the principle of measuring the change in capacitance between the sampling tube and the chassis of the instrument. The sampling tube forms one "plate" and the chassis forms the other "plate" of the capacitor. Atmospheric air is the dielectric. When the liquid sampling tube touches the surface of the liquid to be sampled, the plate of the capacitor that is formed by the sampling tube is now effectively larger because of its contact with the liquid sample. This increases the capacitance between the sampling tube and the chassis that is detected by the liquid level sensing device.
This change in capacitance indicates that the liquid sampling tube has contacted the surface of the fluid sample and this information is sent to the central controller of the instrument for sending control signals to the robotic arm. The control signals tell the robotic arm how much further past the detected surface to lower the fluid sampling tube into the liquid sample for aspirating a predetermined amount of fluid to be analyzed. A known capacitive touch liquid level sensor is supplied by CAVRO Scientific Instruments, Inc., Sunnyvale, Calif., and marketed as Model No. 721014B. The control signals for controlling the robotic arm are received and sent by the central controller of the instrument in a manner well understood by those skilled in the art.
A further attempt to solve the problem of heating a sampling tube without adversely affecting the capacitive touch liquid level sensor is known. In this attempt a water jacket was disposed on the liquid sampling tube for circulating warm fluid to heat up the aspirated sample. However, such a water jacket proved to be unsatisfactory owing to its being bulky, and, hence, not usable in the desired application with an automated liquid sampling apparatus.